The invention relates generally to tomographic imaging and, more particularly, to methods and systems for automatically generating a volumetric image using an adaptive voxel grid.
Tomographic imaging has become an integral part of healthcare services, allowing physicians and radiologists to obtain three-dimensional representations of selected organs or tissues of a patient non-invasively. Tomosynthesis is a variation of conventional planar tomography in which a limited number of radiographic projections are acquired at different angles relative to the patient. In tomosynthesis, an X-ray source produces a fan or cone-shaped X-ray beam that is collimated and passes through the patient to then be detected by a set of detector elements. The detector elements produce a signal based on the attenuation of the X-ray beams. The signals may be processed to produce a radiographic projection, including generally the line integrals of the attenuation coefficients of the object along the ray path. The source, the patient, or the detector are then moved relative to one another for the next exposure, typically by moving the X-ray source, so that each projection is acquired at a different angle.
By using reconstruction techniques, such as filtered backprojection, the set of acquired projections may then be reconstructed to produce diagnostically useful three-dimensional images. Because the three-dimensional information is obtained digitally during tomosynthesis, the image can be reconstructed in whatever viewing plane the operator selects. Typically, a set of slices representative of some volume of interest of the imaged object is reconstructed, where each slice is a reconstructed image representative of structures in a plane that is essentially parallel to the detector plane, and each slice corresponds to a different distance of the plane from the detector plane.
In Digital Breast Tomosynthesis (“DBT”), volume datasets are typically reconstructed with an anisotropic voxel size, where the in-plane voxel spacing within a slice usually reflects the detector pixel size (e.g., 0.1 mm), and the slice separation is generally between 0.5 and 1.0 millimeter (mm). This anisotropic voxel spacing results from a combination of the limited angular range acquisition, workflow considerations (image review time), and data storage considerations. When the overall tomographic angle is increased, slice spacing may need to be reduced to avoid losing (or degrading) fine-scale image detail (e.g., small microcalcifications). That is, a blurring effect and an associated loss in contrast for small microcalcifications may result in a reduced sensitivity of tomosynthesis for the detection of microcalcifications as compared to standard Full Field Digital Mammography (“FFDM”) unless slices with a finer slice spacing are provided. It may be impractical, however, to reduce slice spacing in view of workflow and/or data storage considerations.
It would therefore be desirable to reconstruct a tomosynthesis image in such a way so as to improve image quality (e.g., with respect to small microcalcifications).